This invention relates to commercial display systems and the like, and more particularly, to an improved method and apparatus for lighting such commercial display systems and the like. The invention has particular applications in commercial refrigeration systems used in a retail environment, such as retail display freezers.
Red-Green-Blue (RGB) based white Light Emitting Diode (xe2x80x9cLEDxe2x80x9d) illumination is known in the art and is finding applications in backlighting for LCD panels, lighting for commercial freezers, signage etc. For these applications, linear power supplies or switch-mode power supplies are used to drive the LEDs. The efficiency of the overall system with the use of linear power supply is low and the switch-mode power supply overcomes this problem. Since there are three LED light sources, three independent power supplies are used to drive the LEDs with a proper current control scheme. In this configuration, each power supply may contain independent AC/DC converter, a power factor correction unit, an isolation transformer, and a DC/AC converter system. There exists a redundancy in this scheme due to the three independent AC/DC converters, power factor correction unit, and the isolation transformer. In addition, it requires independent control of the converters in the power supplies. This scheme results in increase in cost, complexity in control and poor performance.
A still further problem with the present state of the art is accurately controlling the amount of each type of light emitted. More specifically, the color of the light resulting from the combination of the light emitted by the red, green, and blue lights is determined largely by the relative amounts of each type of light that gets mixed together. The light source associated with each type of light has a different sensitivity to age and temperature, as well as other factors. As a result, maintaining the appropriate amount of each color of light such that the resultant total light amount is correct is a difficult if not impossible task.
Another issue not addressed by prior systems is the fact that in a display case or retail display refrigeration device, the type and amount of light used to display particular products may influence a consumer""s purchasing decisions. There exists no technique of uniformly assuring that each specific product is displayed using the optimum lighting conditions.
The above and other problem of the prior art are overcome in accordance with the present invention which relates to an LED current driver for a lighting system applicable in commercial displays. In accordance with the invention, drivers are utilized to drive red, green, and blue LEDs in a specified proportion with one another. A feedback loop transmits color and intensity information to a microprocessor, which adjusts the values of each of the red, green, and blue lights to achieve a prescribed lighting intensity and color.
In an enhanced embodiment, a computer and storage are provided for determining the intensity and color of light used based upon specific products being displayed, or specific times of day. Specifically, a computer may adjust the light color and/or intensity to optimize display at particular times or for particular products. In one exemplary embodiment, a microprocessor controlled AC distributed power supply system is used to provide LED drive currents to a white LED luminary for lighting commercial freezers. The AC distributed system contains a front-end AC/DC converter with power factor correction, a high frequency inverter, an isolation transformer and three DC/AC converters with RGB drive current control system. A single, front-end AC/DC converter system converts the AC supply and maintains a constant DC link voltage as the input to the high frequency DC/AC inverter. The AC/DC converter also performs the power factor correction at the AC mains. The high frequency converter converts the DC voltage to AC and supplies powers to three AC/DC converters with LED drive current control.
The power converter system is controlled by a microprocessor system. The microprocessor system provides an integrated closed loop control and the PWM generation for the converter systems, in addition to the control of the white light generated by the LED luminary. This approach provides an integral solution for the control of the LED driver system. The control algorithm for the microprocessor system is developed for modularity and with multi-processing features, to provide the effective controlling capabilities for the microprocessor system.
The microprocessor system is also optionally connected to a user computer, which is programmed with the food that will be displayed in the freezers. The computer in the shop selects the suitable white color point and the lighting level that should be generated by the system when a specified food is being displayed in the freezers, based upon programmed user priorities. The computer supplies this information to the microprocessor system at the appropriate times, which controls the driver system to produce the required color and lighting level. Therefore, the selection of the color and lighting level for the displayed food is automated. The computer can also start and stop the freezer driver such that the freezer lights are switched off automatically when it is not needed, and therefore, the power saving is achieved.
In another enhanced embodiment, the system is arranged to accept data from an input device, such as a hand held keyboard or bar code scanner.